One of the things that make ocean beaches so appealing and exciting is the surf. Also, nothing is more fun than jumping into the water and playing in the surf, feeling the power of the waves crashing ashore. Unfortunately, the unpredictable nature of the surf and its power result in many injuries and deaths each year, even affecting experienced swimmers and surfers.
The thousands of waves that strike a beach every day are generated by the wind. Generally speaking, as the wind speed increases so does the surf. Waves that break on beaches can be locally generated or be spawned thousands of miles away by storms at sea. Hurricanes cause the largest waves, termed swell, along the Atlantic Coast, while migratory low-pressure cells (e.g., storms) at high latitudes generate the great Pacific Ocean swells. The north shore of Oahu, Hi. is directly exposed to these giant ocean swells that can reach 30 feet high during the international surfing contests in January. These huge swell waves are hitting beaches when the weather is perfect—sunny and cloud free.
Wave height is the primary determinant of rip current strength, but wavelength is also significant. Wavelength refers to the width of the wave, which is measured from trough to trough. The height and width determine the volume of water in a wave. Some waves that peak when breaking may appear powerful, but there is no real force behind them without a large mass of water. By contrast, the big swells that dominate the Pacific coast tend to have long wavelengths, making them very powerful waves that break with considerable force.
It is nearly impossible to measure wavelength when in the water, but you can easily count in seconds the time between waves as they break. The greater time between plunging breakers (termed the wave period), the longer their wavelength and consequently the greater the force for a wave of a particular height. Long-period swell waves of around 20 seconds are the best surfing waves along the Southern California Coast, but these turbulent waters are best avoided for swimming; I suggest heading to the nearest heated surf pool.
High waves can be quite dangerous. What is not understood by the public is that the energy is proportional to the height of the wave squared. Therefore, a three-foot wave is nine, not three, times more powerful than a one-foot wave. Onshore breaking waves that exceed five feet are generally too dangerous for bathers and swimmers. Experienced surfers look for the big waves, but good surfing beaches are often dangerous for swimming.
The two primary types of breaking waves are plunging and spilling. Plunging waves are by far the most exciting and dangerous, being characterized by great force and velocity. Plunging breakers are formed when swell suddenly encounters a shallow bottom, such as a reef, large sand bar or steeply-sloping beach. The wave is forced to peak up and break suddenly with all of its force concentrated in a limited area. Plunging waves often generate rip currents and shore breaks on steep beaches and are responsible for many more injuries than spilling or surging waves.
Spilling breakers, which are more common and much less imposing, lose their energy over long distances in contrast to plunging waves. The breaking water rolls or tumbles forward as the wave advances into shallower water, producing a wide surf zone. Spilling waves generally provide safe conditions for waders, swimmers, and boogie boarders; the U. S. East and Gulf coast beaches are most often subject to this type of breaking waves.
What Causes Rip Currents
Waves contain the energy that generates currents at beaches. These currents are the ones that primarily affect bathers and swimmers and extend from the shoreline to the outermost breakers (e.g., the surf zone). Tidal jets are another dangerous current (totally unrelated to waves) that occur locally at inlets or other constrictions; these strong currents are caused by the flooding and ebbing tides.
Wave breaking produces swash—the water that moves up and down a beach face. The sheet of water moving up the beach is called the swash uprush or just uprush. The larger the breaking wave, the deeper and faster moving is the uprush. The swash that does not sink into the sand is then drawn by gravity down the beach as water seeks its own level. When the waves are high and the beach is steep, the swash backwash can be powerful and is sometimes called undertow, which can knock you around, but is seldom a problem except for children.
Undertow, which is strong backwash, only pulls you into, but not beyond the wave breaking on the beach. The effect of the wave breaking over the top of you can give the impression of being sucked under the wave, hence the concept of undertow.
Some beach communities, especially along Southern California and Hawaii, post signs warning swimmers of undertow during big wave days. Undertow is the one thing that many beachgoers have heard about, yet the real danger is rip currents.
On big wave days, especially with large plunging breakers, the swash can be quite strong; the water shoots up the beach face, providing a good ride for boogie boarders. The backwash of the swash is particularly problematic on steeply-inclined beaches near the time of high tide. This return flow of water that is caused by gravity can topple people—it is difficult to maintain your footing in the swift current as you are pulled forcefully toward deeper water. While this current can be overpowering during times of big plunging waves, it will not take you beyond the breaker line (unlike a rip current which carries you offshore through the surf zone). Of course, it can be dangerous if you are pulled into the next large plunging wave that is breaking into shallow water (e.g., shore break conditions).
The most frequently encountered current by bathers and swimmers on ocean beaches is the longshore current, which is produced by waves breaking at an angle to the shoreline. Surf Pools will face these same dangerous rip currents. Anyone who has spent time on surf beaches has experienced this current that moves you along the shore, but not offshore. Sometimes the current is so gentle that you don't even feel it moving you; it is not until you get out of the water to find your towel that you realize its effect. Other times, especially when the breaking waves are coming from an oblique angle to the shoreline and are quite large, this current can feel like a river flow (you really should not be in the water during these conditions). In fact, the longshore current is responsible for huge quantities of sand movement, making beaches a “river of sand.” These same dangerous conditions can also occur in surf pool with big surfing waves in high frequency.
Rip currents are caused by water being pushed up the beach above mean sea level by large breaking waves. Swashes generated by plunging breakers of large swell waves are the most effective in producing the conditions for rip generation. As in the normal swash process, this water that is piled up on the beach is subject to gravity, pulling it back down the slope to the sea surface. Subsequent large breaking waves can continue to pile the water up on the beach, causing a temporary damming effect. Water will follow the path of least resistance, such as an underwater trough or along a groin, in seeking its own level. A concentrated flow of returning water to the ocean becomes a rip current, moving away from the beach toward the offshore.
Rip currents have three components—feeder, neck and head. Oftentimes this mushroom shape is not present or apparent to beachgoers from the vantage point of the water's edge.
The feeder current is the main source of water for the rip. Water that has been pushed and piled up on the beach is often moved along the shore for a short distance by the feeder currents to the underwater channel or trough. Once the water reaches the channel or encounters an obstacle to its along-the-shore movement, it will turn seaward as a rip current.
There may be one or two feeder currents, depending upon the wave approach and prevailing longshore current.
The neck section is where the concentrated flow of water moves from the beach through the surf zone. Current speeds are quite fast, often reaching 2-3 feet per second and measured to be as high as 6 feet per second along some Australian high-surf beaches. The neck of the rip can vary in width from a few yards to tens of yards. The majority of both rescues and drownings occur when people are being pulled offshore in the rip neck. The rip head, which sometimes has the classic mushroom shape, develops where the current has moved beyond the surf zone.
Rip currents typically form in pronounced breaks or “holes” in the nearshore bar or reef, which serve as the conduits for the strong seaward-flowing current. Such strong currents could scour holes in the inner bar or reef.
Artificial surf reefs (ASRs) are structures specifically aimed at modifying the nearshore wave field transformation to improve surfing conditions or surfability. With the increasing popularity of surfing, the demand for such artificial reefs is ever growing. Artificial Surf Reefs (ASRs) are planned to be constructed in big surf pools for indoor and outdoor surfing. Nevertheless, artificial surf reef design is often done fairly ad hoc and there remains great uncertainty as to what the optimal dimensions of the artificial surf reef should be.
In Artificial Surf Reefs, Henriquez (2004) investigated, through a combination of numerical and experimental modelling, how Artificial Surfing Reefs design affects the resulting surfability. The quality of a surf break is generally expressed in three measurable parameters: breaker height, peel angle and breaker shape. Together these parameters determine the surfability of the wave. In particular, the peel angle (a measure related to the rate at which the wave breaks along its crest) is an important measure that plays a dominant role in Artificial Surfing Reefs design. The numerical modelling (Henriquez, 2004) was done without taking into account wave-driven currents. The experimental modelling by Henriquez showed that, approximately 20% of the wave ride was negatively affected by rip currents driven by wave breaking over the ASR. The waves in the rip current were breaking in sections, irregular and with a rough water surface, in other words: unsuitable for surfing.
Over the last ten years numerous Artificial Surf Reefs in surf pools have been designed and a few are actually built. In order to design an artificial surf reef, it is important to understand the basics of ocean wave transformation over topography, including the effects of e.g. shoaling, refraction and diffraction. It is also important to understand waves from a surfer's point of view to understand what kind of wave an ASR should produce. The interaction between the waves and the reef are explained in this current invention.
In previous research (Henriquez, 2004), the currents driven by wave breaking over the reef had not been taken into the design. It appeared that wave-driven currents play an important role in ASR design.
Peel Angle
Surfable waves never break all at once along the wave crest. If this occurs, surfers would say that the waves are closing-out and not suitable for surfing. In order for a wave to be surfable, the wave has to break gradually (read peel) along the wave crest. The velocity with which this happens is called the peel rate.
The peel angle is the most important surfability parameter. The peel angle is the angle α enclosed by the wave crest and the breaker line (In Recreational Surf Parameters, University of Hawaii, Walker, 1974). Also, the wave celerity c and the peel rate Vp vectors are indicated. The absolute value of the vector sum of these velocities is the actual velocity experienced by the surfer, called down-line velocity Vs, which is the magnitude of the velocity vector along the breaker line.
Whether a wave is surfable or not depends mainly on the value of the peel angle α. The down-line velocity is related to the peel angle. Thus, when the peel angle becomes too small, the down-line velocity becomes very high and too fast for the surfer. The value of the peel angle needs to be sufficiently large in order for a wave to be surfable. The velocity a surfer can reach depends, mainly on the wave height H and the skill of the surfer. In, “Classification of surf breaks in relation to surfing skill”, Hutt et al. (2001) investigated what the necessary peel angle has to be for a given wave height H and surfer skill.
The higher the waves, the smaller the peel angle can be; likewise, with increasing surfer skill a smaller peel angle can is acceptable. The definition of these surfer skills and the peel angle related to the skill of the surfer and the wave height are described in Hutt et al. (2001).
The phenomenon of peeling waves is not as obvious as one would think. Waves approaching a sloping shore with straight and parallel depth contours under an angle θ will refract such that the wave angle at breakpoint θb of the waves is nearly zero. The main challenge of an Artificial Surfing Reef is to obtain peel angles which are large enough to be surfable. This can be achieved by using a reef with relatively steep slopes and with an angle β enclosed between the reef normal and the beach normal. The ASR, and thus, wave refraction over the reef, has to start in sufficiently shallow water such that the peel angles can be large enough for surfing purposes. This can be understood by considering Snel's law:
where c is the wave celerity and θ is the wave angle, subscripts b and r denote the breakpoint and the depth at which the reef starts respectively. Snel's law only applies to an alongshore uniform beach and therefore the wave angles must be defined with respect to the reef normal. Then the break angles θb are replaced with the peel angles α and Equation becomes:α=(2.2)With θr constant, variations of the depth at which the reef starts hr only weakly affect cb. Then it follows from Equation 2.2 that the peel angle α is directly related to the wave celerity cr. By decreasing the depth at which the reef starts, the wave celerity cr is decreased resulting in higher peel angles.Wave Height:
Waves can be surfed from 0.15 m up to 25 m high. Long boarders start surfing when waves are 0.5 m, while some professional surfers still surf waves of 25 m. In general, most recreational surfers are surfing waves between 1 m and 3 m. The wave height at the take-off can be increased by the artificial surf reef, using the phenomenon of wave focusing. Wave focusing occurs where wave rays converge due to wave refraction. Due to wave focusing the wave heights along the wave crest have a gradient, the part with high wave heights will break in deeper water than the part with low wave heights, resulting in a breaker line not parallel to the depth contours. This can also affect the peel angles. The effect of wave focusing on the peel angle can only be estimated with the use of numerical models do to the complexity of the combined effects of wave refraction and diffraction.
Breaker Shape:
The shape of a breaking wave is of great importance for surfing. The breaker type is a means of classifying the shape of breaking waves. The main surfable types are:                Spilling breakers occur if the wave crest becomes unstable and flows down the front face of the wave producing a foamy water surface. Surfers would say a ‘weak’ wave.        Plunging breakers occur if the crest curls over the front face and falls into the base of the wave, resulting in a high splash. Surfers call this a ‘tubing wave’.        Collapsing breakers, these breaking waves occur if the crest remains unbroken and the front face of the wave steepens and then falls, producing an irregular turbulent water surface-surfers often encounter this regime at reef breaks when the tide is too low and the reef is not submerged enough to produce surfable waves, and so it is an unsurfable regime.        Surging breakers these waves occur if the crest remains unbroken and the front face of the wave advances up the beach with minor breaking. His regime is also unsurfable.        
Currents around a surf break are of vital importance when considering the surfability of the break. Rip-currents, narrow strong currents that move seaward through the surf zone negatively affects good surfable waves. When the rip-current flows through the breakers, the wave appears to get a rough surface and breaks in an irregular manner, making the waves unsuitable for surfing. Rip-currents can be advantageous as well; the surfer can use the rip-current to get easily outside the breaker zone. It can also be the case that the waves are perfectly surfable, but yet unreachable due to strong currents.
It is observed by Henriquez (2004) in his experiment that waves in a rip current break irregular and in sections. This might be caused by variations in the velocity of the rip-current. These variations in wave heights can be the cause of the irregular and in sectional breaking of the waves. It turns out that for the conventional Artificial Surfing Reef rip currents negatively affect approximately 20% of the wave ride. Thus in order to design an improved Artificial Surfing Reef the wave-driven currents over the reef have to be taken into account. In other words, the currents which are flowing through the breakers have to be minimized. Therefore, it is important to understand the driving mechanism of the wave-driven currents over the conventional ASRs.
The main driving mechanisms for the rip currents through the breakers caused by the artificial reef are the currents induced by differences in pressure gradients. These pressure gradients occur due to differences in breaker heights over the reef and at the sides of the reef. The rip currents are also driven by the along shore currents.